Surfaces for collectors of solar radiation



H. z. TABoR 3,129,703

suRFAcEs FoR COLLECTORS oF soLAR RADIATION April 21, 1964 Filed Aprilll, 1960 F/GZ.

Inventor Attorney United States Patent Office ,gem APQ 112i 3,129,703SURFACES FOR CGLLECTORS OF SOLAR RADIATION Harry Zvi Tabor, BethHakerem, Jerusalem, Israel, assgnor to The Government of Israel onbehalf of the State of Israel, Jerusalem, israel Filed Apr. 11, 196i),Ser. No. 21,257 Claims priority, application Israel Apr. 16, 1959 4Claims. (Cl. 126-270) The present invention relates to solar heaters,and particularly to solar heaters of the type generally referred to ashaving receivers which absorb impinging solar radiation and convert itinto utilizable heat.

Solar heaters generally comprise a receiver, and a heat transfer bodywhose function is to absorb heat and transfer the absorbed heat to aheat transfer medium, e.g. water, which may pass to a heat storagesystem, such as a hot water boiler.

In accordance with U.S. Patent No. 2,917,817 receivers for solar heatersare known which are composite bodies comprising a metal base and a thincoating applied to the base in heat-conducting relation, the physicalproperties of the base and coating in their behaviour to light and heatbeing so selected that the coating is consequently not heat-absorbing,substantially no heat is radiated from the base through the coating andthe receiver as a whole is light-absorbing and substantially notheat-emissive.

These reecivers are selective in their behaviour towards impingingradiation in that they are capable of absorbing visible radiation,converting it into heat and retaining the latter because of their verylow hea-t emissivity. In other words, these receivers are black inregard to visible radiation and white in regard to heat radiation.

The formation of a surface of such specic optical properties calls forspecial methods which require high skill and considerable technicalknow-how. As the receiver is as a rule an integral part of theheat-transfer body of which it forms merely the upper surface, it isnecessary during production to submit the entire heat transfer body tothe handling and treatment required for applyinr the coating in order toproduce the above selective receiver. This means, for example, that theentire body has to be dipped into an electrolytic bath in case thecoating is formed electrolytically, or into a coating solution in casethe coating is formed chemically. The necessity to handle the entireheat transfer body of the solar heater for coating one of its surfacesis in itself a serious disadvantage which becomes all the morepronounced because as a rule the manufacturers of solar heaters lack theequipment and technical knowledge necessary for forming the coating.

The optical phenomena referred to above which lead to the absorption ofvisible light, its conversion into utilizable heat and the retention ofthe latter, all occur on the surface which comprises the coating whilethe remainder of the base has mainly the function of transferring theheat to a heat-transfer medium such as water, oil or the like, wherebythe base serves as merely a heat transfer body.

The present invention contemplates that the receiverheat transfer unitof a solar heater can be made of two parts, viz. an upper one on whichoccurs the optical phenomena and Which may be comparatively thin, and alower one which serves for heat transfer.

The invention is directed to a solar heater of the kind in which thereceiver is a composite body comprising a metal base and a thin coatingapplied to the base in heatconducting relation, the physical propertiesof the base and coating in their behavior -to light and heat being soselected that the coating is substantially not heat absorbof very thickfoils.

ing, so that substantially no heat is radiated from the base through thecoating and the receiver as a whole is light absorbing, andsubstantially not heat emissive, said receiver being characterized inthat the base of the receiver is a thin, metallic foil which is appliedto the heat transfer body in a heat-conducting manner.

The foil may be made entirely of metal, or may consist of a metallizednon-metallic substance, for example, a synthetic plastic sheet material.For the purposes of the present invention all these foils are consideredand referred to as a metallic foil.

The foil has to be stable under operating conditions, i.e. underprolonged heating. It has been found that aluminium and nickel are themost suitable materials though other metals such as copper,copper-nickel alloys, stainless steels, bronzes and the like can beused. The thickness of the foil is not critical. However, very thinfoils are ditiicult to handle while it is difcult to ship rolls Inpractice, a suitable thickness is from 0.02 to 0.25 mm.

As a rule the foil will be applied to the main body by means of aheat-conducting adhesive layer stable at the operating temperatures.Generally it is diflicult to apply a foil of considerable surface areato a base without trapping a certain amount of air between the foil andbase. When the foil becomes hot the trapped air expands and producesblisters on the foil which form areas where the thermal contact betweenthe foil and the receiver surface is greatly reduced. Since good thermalcontact is required, such blisters must be reduced to negligibleproportions and according to a preferred embodiment of the invention,this is obtained by perforating the foil and/or the heat transfer bodywith small holes, such as pin pricks at intervals of about a centimeter.

The invention also contemplates providing sheets adapted to be appliedas receivers to heat transfer bodies of solar heaters, comprising ametallic foil and a thin coating applied to one of its faces in aheat-conducting relation, the properties of the foil and coating intheir behavior to light and heat being so selected that the coating issubstantially not heat absorbing, so that substantially no heat will beradiated from the foil through the coating while the sheet as a Whole islight-absorbing and substantially not heat emissive.

According to a preferred embodiment these sheets are perforated withvery small holes, e.g. pin pricks, at intervals of one centimeter or so.

In accordance with the invention, the manufacture of the solar heaterand the preparation of the radiation-absorbing surface are separateoperations. The sheets according to the invention can be prepared inspecial factories, shipped n the form of rolls, sold according to lengthand applied to any conventional solar heater.

The adhesive layer connecting the sheet and the base must be uniform andaccordingly has to be applied by suitable coating devices either to thetop face of the heat transfer body of the receiver or to the back of thesheet itself. As a rule the later alternative will be preferred forpractical reasons, since factories equipped for the application of theoptical coating are as a rule also able to adapt their equipment to thecoating of the sheets with an adhesive, while factories specialized inthe manufacture of solar heaters are as a rule not so equipped,

The invention is illustrated, by way of example only, in theaccompanying drawings in which:

FIG. l is a fragmentary cross section of a receiver for solar heaters inaccordance with the invention; and

FIG. 2 is a side elevation, partly in longitudinal section, of a sheetaccording to the invention wound for shipment.

The receiver illustrated in FIG. l comprises a heat transfer body 1traversed by pipes 2 for the circulation of the heat exchange uid, eg.water, oil or the like. Pipes 2 are so disposed that their upper section2 is flush with the upper face 3 of body 1. To the face 3 there isapplied, by means of an adhesive layer 6 a foil 4 of aluminium, nickelor any other suitable metal, to whose upper face a coating is applied.The coating is so constituted that the optical behavior of foil 4 andcoating 5 together is as specied hereinbefore. The coating S and foil 4are perforated with a large number of small holes 7 for the escape ofthe air trapped between the foil and base.

FIG. 2 illustrates a roll of a sheet according to the invention in theform in which it is shipped. The sheet comprises a metal foil 4a, oneface of which bears the optical coating Sa, while its back is coatedwith an adhesive layer 6a. The foil la and coating i-o are perforatedwith a large number of small holes 7a. Advantageously the adhesive layeris protected by a release backing of paper or the like (not shown).

An optical coating having the desired properties and methods of applyingthe same to the foil is described in U.S. Patent No. 2,917,817. Forfoils which can easily be rolled up, the application of the' coating isconveniently carried out in a continuously operating coating orelectroplating machine designed for the purpose. Thus, while thereceivers of the solar heaters according to the present invention arecoated according to the teachings of US. patent specification No.2,917,817, the present invention is an improvement of the receiveraccording to said patent in that it provides a novel stratifiedstructure of the receiver which is of great practical advantage and willbecome apparent further below.

Adhesives stable at elevated operating temperatures are readilyavailable. An adhesive which is either free of solvents or pre-driedbefore the two surfaces are brought together, is preferred in order tominimize the formation of blisters. A suitable adhesive is for examplethat known as No. 269, manufactured by the Dow Corning Corporation ofthe U.S.A., rnade by dissolving dimethyl polysiloxane in xylene andbeing a pressure sensitive adhesive which is dried after application toone surface of the foil by heating to G-200 C. for 5-15 minutes. Theadhesive layer is made as thin as practicable, e.g. of the order of 0.1mm. or less. The temperature drop between the foil and heat transferbody has been found to be only a few degrees when the heater isirradiated by noon sunshine. Even though such adhesives may be chosen asproduce very little vapor after the initial drying process, it is noteasy to avoid the trapping of air bubbles when a large area of foil isapplied to the base,

but in this case the perforations 7 as referred to above, provide anescape for the trapped air. 1n some cases it is found Convenient toapply a pressure roller to the heated nished assembly in order to ensureexpulsion of air or vapor through the perforations.

I claim:

1. A solar heater comprising a receiver constituted by a thin exiblemetallic foil and means delining a surface coating on said foil in faceto face heat conducting relation therewith, the receiver being eXibleand adapted for being wound in a roll, said heater fur-ther comprising aheat transfer body, and adhesive means joining the receiver in heatconducting relationship to the heat transfer body, said surface coatingbeing light absorptive, heat transparent and non radiant oi' heat toconfer a substantially low heat emissivity for said receiver in orderthat impinging light energy is absorbed in said coating and istransferred in the form of heat to said foil with comparatively smalllosses of heat by re-radiation from said foil, said foil, by virtue ofthe heat conducting relation established with the body being operativeto transfer heat to said heat transfer body whereat accumulation of heatmay be effected while said receiver prevents emission of heat from saidbody.

2. A heater as claimed in claim 1 wherein the foil is constituted ofaluminum.

3. A heater as claimed in claim 1 wherein the foil is constituted ofnickel.

4. A heater as claimed in claim 1 wherein the foil and coating areprovided with a plurality of coincident holes.

References Cited in the le of this patent UNITED STATES PATENTS2,402,737 Delano June 25, 1946 2,788,316 Bjorksten Apr. 9, 19572,843,536 Mount July 15, 1958 2,850,404 Dunlap Sept. 2, 1958 2,889,233Stetey June 2, 1959 2,917,817 Tabor Dec. 22, 1959 2,998,006 lohnstonAug. 29, 1961 3,000,375 Golay Sept. 19, 1961 FOREIGN PATENTS 821,237Great Britain Oct. 7, 1959 OTHER REFERENCES Solar Heating for SwimmingPools, New Zealand Engineering, September 15, 1956, pages 298-299.

1. A SOLAR HEATER COMPRISING A RECEIVER CONSTITUTEDD BY A THIN FLEXIBLEMETALLIC FOIL AND MEANS DEFINING A SURFACE COATING ON SAID FOIL IN FACETO FACE HEAT CONDUCTING RELATION THEREWITH, THE RECEIVER BEING FLEXIBLEAND ADAPTED FOR BEING WOUND IN A ROLL, SAID HEATER FURTGHER COMPRISING AHEAT TRANSFER BODY, AND ADHESIVE MEANS JOINING THE RECEIVER IN HEATCONDUCTING RELATIONSHIP TO THE HEAT TRANSFER BODY, SAID SURFACE COATINGBEING LIGHT ABSORPTIVE, HEAT TRANSPARENT AND NON RADIANT OF HEAT TOCONFER A SUBSTANTIALLY LOW HEAT EMISSIVITY FOR SAID RECEIVER IN ORDERTHAT IMPINGING LIGHT ENERGY IS ABSORBED IN SAID COATING AND ISTRANFERRED IN THE FORM OF HEAT TO SAID FOIL WITH COMPARATIVELY SMALLLOSSES OF HEAT BY RE-RADIATION FROM SAID FOIL, SAID FOIL, BY VIRTUE OFTHE HEAT CONDUCTING RELATION ESTABLISHED WITH THE BODY BEING OPERATIVETO TRANSFER HEAT TO SAID HEAT TRANSFER BODY WHEREAT ACCUMULATION OF HEATMAY BE EFFECTED WHILE SAID RECEIVER PREVENTS EMISSION OF HEAT FROM SAIDBODY.